Two science teachers who have spent the past five years under NASA's tutelage are about to graduate with high-flying honors.

The space shuttle flight Wednesday night of Joseph Acaba and Richard Arnold II will mark the first time two one-time teachers have rocketed into space together. And during the two-week construction mission to the international space station, both will attempt multiple spacewalks — the most dangerous job in orbit.

The flight on shuttle Discovery was delayed a month because of concerns about hydrogen gas valves in the engine compartment. After extra tests, NASA deemed the spacecraft safe to fly.

Discovery's astronauts arrived at the launching site Sunday afternoon, four hours ahead of the start of the countdown, and thanked everyone who helped resolve the valve issue.

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The teachers and their five crewmates — the usual assortment of military pilots and rocket scientists — will deliver and install a final set of solar wings for the space station.

With just over a year remaining until the orbiting complex is completed, the framework holding the solar wings is the last major American-made building block left to fly.

This flight comes a year and a half after the last teacher-astronaut, Barbara Morgan, went into space after a two-decade wait.

Morgan was the backup in the mid-1980s for schoolteacher Christa McAuliffe, who was killed when space shuttle Challenger exploded after takeoff.

Acaba was a freshman at the University of California at Santa Barbara when McAuliffe died on Jan. 28, 1986. Arnold was fresh out of college and living in Washington, and his wife-to-be was a student-teacher.

"It definitely had an impact when you look at the sacrifices that she [McAuliffe] made and the importance that NASA put on it," Acaba said. When it came time for him to step up, "it really made you feel like you were doing something worthwhile."

NASA didn't pair the two space rookies — Joe and Ricky to their friends — because they were teachers. Each had skills that were deemed essential for this flight.

For instance, both worked in the space station branch at Johnson Space Center in Houston, dealing with hardware and technical issues.

Besides setting up the new solar wings, the astronauts will deliver a spare urine processor for the space station's balky water recycling system, tackle some maintenance work, and drop off astronaut Koichi Wakata. The Japanese Space Agency astronaut will live there for at least three months.

Discovery's other astronauts are commander Lee Archambault, Dominic "Tony" Antonelli, Steven Swanson and John Phillips.

The mission will be so busy that NASA is keeping education-type events to a minimum. Channel One News, a newscast for teenagers, will interview Acaba and Arnold during the flight with questions coming from students' submissions.

Acaba, 41, who's from Anaheim, Calif., is a one-time geologist and Peace Corps volunteer who served in the Marine Corps Reserves. The first person of Puerto Rican heritage to go into space, he'll carry that territory's flag with him.

Arnold, 45, originally from Bowie, Md., is a trained marine and environmental scientist. Both were part of NASA's first educator-astronaut group chosen in 2004, a year after the shuttle Columbia tragedy that killed seven astronauts.

More teachers with math or science backgrounds are expected in the next class of astronauts this spring and will receive the same training as everyone else. NASA made that the practice in 1998, when Morgan was invited to became a full-fledged astronaut. She finally made it to space in 2007.

In the mid-1980s, McAuliffe and Morgan — who has returned to education and is no longer with NASA — had minimal astronaut training.

The two professions are more alike than one might think, according to Acaba.

"Teachers have to think on their feet. They have to adjust all the time, and I think that's part of what we do" as astronauts, Acaba said. "We train for specific things, but you never really know what's going to happen."

Arnold still sees himself more as a teacher than an astronaut. He's taught around the world from Morocco to Indonesia.

"I guess if you look at it mathematically, I spent 15 years teaching and I'm coming up on five years as an astronaut," Arnold said. "I haven't morphed into an engineer yet, and I'm probably not going to."

For Jane Ashman, principal at central Florida's Dunnellon Middle School, where Acaba taught math and science for four years, the teachers' presence on the flight sends a powerful message to students.

"You can achieve your dream, whatever it is," Ashman said. "You can be anything you want."